For what its worth, my experience has been that birdsnest have a very low tolerance for parameter changes. Though they are considered easy to keep as once settled, they grow like weeds in almost any condition, the abrupt change in any condition can send them over the edge.
My guess is your trimming of the chaeto could have abruptly changed your nutrient levels and caused an RTN event that caused it to wither away so quickly.
I have a tank full of acros and I only kept one birdsnest in the whole tank. Any abrupt changes would send it into a frenzy but all the acros didn't bat an eyelash.
Fortunately it grows like a weed so trim any live pieces like what you did and let it grow out again. Here was mine before I removed it from the system.